Sometimes you have to write a paper, or do a debate, with a position you don't personally agree with. There -were- reasons to defend slavery. They weren't good reasons, but there were reasons!
First of all it was seen as an economic necessity. Cotton was the US's biggest industry in those days, the biggest moneymaker, especially overseas, where it brought a lot of money into the country. Under the plantation system growing cotton required slaves. It was very labor-intensive in those days.
Secondly, Southerners saw slavery as justified, even recommended by the Bible. Both the Mosaic Law and the New Testament have passages justifying slavery and telling slaves to be loyal and faithful to their owners and telling the owners not to mistreat them. Christians in the South believed they were -saving- blacks because bringing them here allowed them to learn about Jesus so they could be saved. Plus it allowed them to do useful work, which they believed was important to salvation (the Devil finds work for idle hands!)
At that time, slavery had been done all through history. If the ancient Romans and Greeks and Egyptians did it, why couldn't we? It was a 'natural' thing, like eating and pooping.
Slavery was ensconced in the Constitution. The Const. specifically mentioned it and allowed it. It counted slaves as 3/5 of a person for purposes of congressional apportionment (but they couldn't vote). It specifically prevented any abolition of slavery for 30 years (based on a compromise). What could be more American than something in the Constitution.
Finally there's the 10th amendment which says that powers not expressly given to the fed. govt. would be given to the states and 'the people'. So there was a states' rights argument that the fed govt. didn't have the power to abolish slavery. Just as 100 years later southerners insisted that the fed. govt. didn't have the power to ban systematic discrimination and segregation.
I don't agree with any of these arguments, but these were the arguments given for slavery.